1. Field of the Invention
The invention relates to lubricant compositions. More particularly, the invention relates to lubricant additives that are soluble with a wide variety of hydrocarbon oils.
2. Background of the Art
Lubricant compositions are widely used in devices with moving mechanical parts, in which their role is to reduce friction between the moving parts. This reduction may, in turn, reduce wear and tear and/or improve the device's overall performance. In many applications lubricant compositions also serve related and non-related supplemental purposes, such as reducing corrosion, cooling components, reducing fouling, controlling viscosity, demulsifying, and/or increasing pumpability.
Most lubricant compositions today include a base oil. Generally this base oil is a hydrocarbon oil or a combination of hydrocarbon oils. The hydrocarbon oils have been designated by the American Petroleum Institute as falling into Group I, II, III or IV. Of these, the Group I, II, and III oils are natural mineral oils. Group I oils are composed of fractionally distilled petroleum which is further refined with solvent extraction processes to improve properties such as oxidation resistance and to remove wax. Group II oils are composed of fractionally distilled petroleum that has been hydrocracked to further refine and purify it. Group III oils have similar characteristics to Group II oils, with Groups II and III both being highly hydro-processed oils which have undergone various steps to improve their physical properties. Group III oils have higher viscosity indexes than Group II oils, and are prepared by either further hydrocracking of Group II oils, or by hydrocracking of hydroisomerized slack wax, which is a byproduct of the dewaxing process used for many of the oils in general. Group IV oils are synthetic hydrocarbon oils, which are also referred to as polyalphaolefins (PAOs).
In order to modify properties of the various base oils, so-called additive packages are frequently employed. Such may include materials designed to serve as antioxidants, corrosion inhibitors, antiwear additives, foam control agents, yellow metal passivators, dispersants, detergents, extreme pressure additives, friction reducing agents, and/or dyes. It is highly desirable that all additives are soluble in the base oil. Such solubility is desirably maintained or maintainable across a wide range of temperature and other conditions, in order to enable shipping, storage, and/or relatively prolonged use of these compositions. It is also highly desirable that the additives offer good environmental performance. This implies that such are not required to carry any hazard classification warning label, and/or are biodegradable and non-toxic to aquatic organisms. However, attainment of these desirable qualities should not be at the expense of overall performance. Unfortunately, many additives that include, as at least one benefit, improved friction reduction suffer from low solubility, poor environmental performance, or both.
Those skilled in the art have attempted to identify friction reduction additives (herein termed “lubricant additives”) that may be included in lubricant compositions with base oils and that do not pose problems relating to both solubility and the environment. One approach to this problem has been to include one or more co-base oils, such as synthetic esters or vegetable oils, in the lubricant composition. For example, esters have been used as co-base oils with polyalphaolefins for this purpose. Unfortunately, such esters often suffer from poor hydrolytic stability, and thus may represent an unacceptable sacrifice in overall performance in order to achieve solubility and environmental acceptance.
Another approach to the problem has been to use lubricant additives containing zinc, sulfur, and/or phosphorus. While these lubricant additives often offer both desirable friction reduction and supplemental properties, such as corrosion resistance, they may be non-biodegradable and/or toxic to the environment. They also tend to be relatively expensive. Examples of these additives may include amine phosphates, phosphate esters, chlorinated paraffinics, zinc dialkyldithiophosphates, zinc diamyldithiocarbamate, and diamyl ammonium diamyldithiocarbamate.
Still another approach has been to use lubricant additives that are polyalkylene glycols, or “PAGs.” Many PAGs are based on ethylene oxide or propylene oxide homopolymers, and are in some cases ethylene oxide/propylene oxide co-polymers. They often offer good performance and environmental properties, including good hydrolytic stability, low toxicity and biodegradability, high viscosity index values, desirable low temperature properties, and good film-forming properties. Unfortunately, they are generally not soluble in hydrocarbon base oils. In particular, their solublility with polyalphaolefins (Group IV oils) is particularly low. Those skilled in art therefore continue to search for polyalkylene glycols that have improved oil solubility in order to take advantage of their many benefits while minimizing the likelihood of environmental problems.